In laser recording it is desired to mark a light or heat sensitive material with a well defined mark. This is particularly important in the field of graphic arts and in particular in exposing of halftone materials such as printing plates. The amount of ink carried by a printing plate is related to the size of the marks making up the image on the plate. The more abrupt the transition of the laser spot marking the material the more consistent the mark size is. Theoretically, for an infinitely sharp transition at the edge of the laser spot the mark size will not change even if the marking threshold or laser energy changes. Since most recording methods require continuous relative motion between the laser spot and the material being marked, the sharpness of the transition in the laser energy forming the mark is affected by the smearing of the edge of the mark. This is known as "motion blur" and the problem is similar to the blur occurring when taking a picture of a moving object with a camera having a long exposure time. The same way that the camera problem can be solved by using a strobe light, the laser recording motion blur can be solved by pulsing the laser. This solution, however, is not practical as it lowers the energy delivered by the laser. It should be noted that even when a high contrast material is being used and the motion blur is not evident by looking at the recorded mark, the negative effects of this blur on the stability and uniformity of the mark are still there.
The problem of lost sharpness caused by motion blur becomes evident by referring to FIGS. 1a and 1b, which shows the prior art. A laser spot S having sharp (i.e. nearly square) transitions 1 is imaged for a time on a moving material 3 by lens 2 to make a mark on material 3. At any instant the energy profile of the imaged spot 4 has sharp transitions similar to the transitions 1 of spot S (within the limitations imposed by the performance of lens 2). The accumulated energy forming the mark is the sum of multiple profiles, spread over an area of the mark. At the instant at which writing the mark begins the energy is distributed as shown by curve 5 and at the instant at which writing the mark ends the energy is represented by curve 4, as material 3 has moved. Summing all these profiles gives the total energy curve 6, which no longer has a sharp transition.
Referring now to FIG. 1b, the size of the mark made on material 3 will depend on the location of the marking threshold relative to curve 6. Where material 3 has a threshold at the energy level indicated by line 7 the mark size is "a". While the mark size will change to "b" if material 3 has a threshold at the energy level indicated by line 8. Both mark "a" and "b" can be very sharp, if material 3 is high contrast material, but the size of the mark will not be stable.
All prior art exposure systems, even those which use square spots, are subject to this deficiency. An example of using square spots to image half-tone images is shown in U.S. Pat. No. 4,025,189 and the triangular profile resulting from a square spot is shown in FIGS. 7-c to 7-e of that patent. Other methods of forming a square laser spot are disclosed in PCT patent application WO 95/18984. This disclosure teaches the generation of a square laser spot but cannot generate an exposure function on the material having a square energy profile due to the "motion blur". It is an object of the invention to generate a square mark created by a square energy distribution, having a sharp transition of the exposing energy at all four sides of the square mark. This square mark can serve as a basic picture element, or pixel, for the construction of images, and in particular half-tone images as used in the graphic arts. By making the size of the square equal to the resolution of the image (i.e. no overlap between squares) the problem known as "dot gain" in graphic arts is eliminated. Square spots created according to the invention have a high degree of immunity to any changes in the imaging process. It is another object of the invention to image light or heat sensitive material in a way requiring a minimum of calibration, particularly when half-tone images are being created.